Once the Elan Programming Environment has found a semantic error, it enters backtrace-mood, displaying Backtrace command, please... on the right of the status line, with an error description on the left of the status line. The backtrace-mood offers to the user the opportunity of unfolding the static nesting of the present unit and listing it. After each unit listed, the Elan Programming Environment waits for the next wish of the user, which can be one of the following:
modify the definition containing this unit and return to the command-mood.
continue backtracing; return to command-mood if current unit is root of program, otherwise display unit immediately surrounding current unit.
show the definition and the value of a given name.
stop backtracing, return to command-mood.
This mechanism serves to pinpoint semantic errors in the program.